Fourier transform Raman lidar for trace gas detection and quantification
- Coleman Research Corp., Huntsville, AL (United States)
The Raman technique, while a valuable tool in chemical and combustion research, is limited in many remote sensing applications because of the low Raman scattering cross-section, which may be three to five orders of magnitude below the Rayleigh (elastic) values. Two concepts for increasing the signal level are discussed. First, use a range-gated Fourier transform spectrometer to increase the system throughput and allow multiplexing advantages. The spectrum is obtained by performing a FFT on the resulting interferogram. Second, since the cross section goes as the fourth power of the optical frequency, use ultra-violet laser illumination, and separate the resulting fluorescence radiation by placing a known dispersion on the transmitted waveform. The techniques for achieving this function, and the mathematical formulation for the phase-modulated auto-correlation which result, are not evaluated in this paper. However, the approach does not appreciably lower the available resolution because the limits are imposed by the sampling function inherent to the finite-duration Michelson mirror scan. A conceptual design using a long-pulse, flashlamp-pumped dye laser is shown, and typical performance equations in the detection of Freon 12, CCl{sub 2}F{sub 2}, are presented. For a one joule laser and a thirty (30) cm aperture operating in darkness, a concentration of 10{sup 23} molecules/m{sup 3} can be detected in a 60 km visibility at a range of 3.4 km. Much greater performance is obtained against molecules exhibiting the resonance Raman effect, such as nitrogen dioxide.
- OSTI ID:
- 227978
- Report Number(s):
- CONF-940723-; ISBN 0-8194-1590-1; TRN: IM9622%%255
- Resource Relation:
- Conference: Annual meeting of the Society of Photo-Optical Instrumentation Engineers, San Diego, CA (United States), 24-29 Jul 1994; Other Information: PBD: 1994; Related Information: Is Part Of Optical spectroscopic techniques and instrumentation for atmospheric and space research; Wang, J.; Hays, P.B. [eds.]; PB: 662 p.; Proceedings/SPIE, Volume 2266
- Country of Publication:
- United States
- Language:
- English
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